feat(spanner): add autoscaling config to the instance proto

protos/google/spanner/admin/instance/v1/spanner_instance_admin.proto

@@ -562,6 +562,66 @@ message InstanceConfig {
   State state = 11 [(google.api.field_behavior) = OUTPUT_ONLY];
 }
 
+// Autoscaling config for an instance.
+message AutoscalingConfig {
+  // The autoscaling limits for the instance. Users can define the minimum and
+  // maximum compute capacity allocated to the instance, and the autoscaler will
+  // only scale within that range. Users can either use nodes or processing
+  // units to specify the limits, but should use the same unit to set both the
+  // min_limit and max_limit.
+  message AutoscalingLimits {
+    // The minimum compute capacity for the instance.
+    oneof min_limit {
+      // Minimum number of nodes allocated to the instance. If set, this number
+      // should be greater than or equal to 1.
+      int32 min_nodes = 1;
+
+      // Minimum number of processing units allocated to the instance. If set,
+      // this number should be multiples of 1000.
+      int32 min_processing_units = 2;
+    }
+
+    // The maximum compute capacity for the instance. The maximum compute
+    // capacity should be less than or equal to 10X the minimum compute
+    // capacity.
+    oneof max_limit {
+      // Maximum number of nodes allocated to the instance. If set, this number
+      // should be greater than or equal to min_nodes.
+      int32 max_nodes = 3;
+
+      // Maximum number of processing units allocated to the instance. If set,
+      // this number should be multiples of 1000 and be greater than or equal to
+      // min_processing_units.
+      int32 max_processing_units = 4;
+    }
+  }
+
+  // The autoscaling targets for an instance.
+  message AutoscalingTargets {
+    // Required. The target high priority cpu utilization percentage that the
+    // autoscaler should be trying to achieve for the instance. This number is
+    // on a scale from 0 (no utilization) to 100 (full utilization). The valid
+    // range is [10, 90] inclusive.
+    int32 high_priority_cpu_utilization_percent = 1
+        [(google.api.field_behavior) = REQUIRED];
+
+    // Required. The target storage utilization percentage that the autoscaler
+    // should be trying to achieve for the instance. This number is on a scale
+    // from 0 (no utilization) to 100 (full utilization). The valid range is
+    // [10, 100] inclusive.
+    int32 storage_utilization_percent = 2
+        [(google.api.field_behavior) = REQUIRED];
+  }
+
+  // Required. Autoscaling limits for an instance.
+  AutoscalingLimits autoscaling_limits = 1
+      [(google.api.field_behavior) = REQUIRED];
+
+  // Required. The autoscaling targets for an instance.
+  AutoscalingTargets autoscaling_targets = 2
+      [(google.api.field_behavior) = REQUIRED];
+}
+
 // An isolated set of Cloud Spanner resources on which databases can be hosted.
 message Instance {
   option (google.api.resource) = {
@@ -606,8 +666,12 @@ message Instance {
   string display_name = 3 [(google.api.field_behavior) = REQUIRED];
 
   // The number of nodes allocated to this instance. At most one of either
-  // node_count or processing_units should be present in the message. This
-  // may be zero in API responses for instances that are not yet in state
+  // node_count or processing_units should be present in the message.
+  //
+  // Users can set the node_count field to specify the target number of nodes
+  // allocated to the instance.
+  //
+  // This may be zero in API responses for instances that are not yet in state
   // `READY`.
   //
   // See [the
@@ -616,14 +680,26 @@ message Instance {
   int32 node_count = 5;
 
   // The number of processing units allocated to this instance. At most one of
-  // processing_units or node_count should be present in the message. This may
-  // be zero in API responses for instances that are not yet in state `READY`.
+  // processing_units or node_count should be present in the message.
+  //
+  // Users can set the processing_units field to specify the target number of
+  // processing units allocated to the instance.
+  //
+  // This may be zero in API responses for instances that are not yet in state
+  // `READY`.
   //
   // See [the
   // documentation](https://cloud.google.com/spanner/docs/compute-capacity)
   // for more information about nodes and processing units.
   int32 processing_units = 9;
 
+  // Optional. The autoscaling configuration. Autoscaling is enabled if this
+  // field is set. When autoscaling is enabled, node_count and processing_units
+  // are treated as OUTPUT_ONLY fields and reflect the current compute capacity
+  // allocated to the instance.
+  AutoscalingConfig autoscaling_config = 17
+      [(google.api.field_behavior) = OPTIONAL];
+
   // Output only. The current instance state. For
   // [CreateInstance][google.spanner.admin.instance.v1.InstanceAdmin.CreateInstance],
   // the state must be either omitted or set to `CREATING`. For